Efficient in situ reduction and recovery of neptunium(V) using redox-functionalized deep eutectic solvents

The separation and recovery of neptunium (Np) represents a critical challenge in nuclear waste reprocessing. To address the difficulty of effectively capturing Np(V) in conventional extraction systems, this study designed and constructed three hydrophobic deep eutectic solvents (DESs) using tri-n-octylphosphine oxide (TOPO) as the hydrogen bond acceptor (HBA) and dihydroxybenzene isomers (catechol, CC; resorcinol, RC; hydroquinone, HQ) as hydrogen bond donors (HBD). These DESs enabled the in situ reduction at the liquid-liquid interface and highly efficient extraction of Np(V). Among them, the TOPO-CC DES system exhibited outstanding extraction performance for Np(V) over a broad acidity range of 10^-3 to 2.0 M HNO₃, achieving a single-stage distribution ratio (D_Np) up to 85 and an in situ reduction conversion ratio from Np(V) to Np(IV) exceeding 98% within 5 minutes. Mechanistic investigations revealed that the extraction process follows a "coordination-driven reduction" mechanism. TOPO initially forms a weak coordination intermediate with Np(V) at the interface, thereby lowering its reduction potential. Subsequently, the dihydroxybenzene component facilitates efficient electron transfer within the organic phase, reducing Np(V) to Np(IV), which is strongly complexed and extracted by TOPO, completing an integrated "coordination-reduction-extraction" process. Furthermore, this system maintained stable extraction performance under challenging conditions, including high salinity (1-5 M NaNO₃), a wide temperature range (293-313 K), and significant cumulative γ irradiation doses (up to 60 kGy). This study provides a novel integrated DES strategy for the efficient and green recovery of Np(V) and lays a scientific foundation for the application of functionalized deep eutectic solvents in advanced nuclear fuel cycles.